One type of ion storage device, such as a quadrupole mass filter, contains ions in the radial direction while allows them to move or drift in the axial direction. Another type of ion storage device, such as a three-dimensional quadrupole ion trap, contains ions in a certain spatial area. In any type of storage devices, they include plural electrodes on which an appropriate radio frequency (RF) voltage is applied to form a quadrupole electric field in the space surrounded by the electrodes. Owing to the quadrupole electric field, ions are contained or stored in the space. The kinetic state of the ions is different depending on their mass to charge ratios, which is used to discriminate or dissociate ions.
In still another type of ion storage devices, a multipole electric field is generated to form a broader ion storing space, whereby a larger number of ions can be stored. The type includes an ion storage device using octapole rods provided with a pair of ion-reflecting end electrodes.
Such an ion storage device may be used as a mass spectroscope by itself, or it may be used as an ion pre-processing device for a subsequent ion analyzer. For example, in a quadrupole mass spectrometer, a pre-filter is placed before a quadrupole mass filter to enhance the ion introducing efficiency from the ion source. In a tandem quadrupole mass spectrometer, plural quadrupole mass filters are serially placed to perform a multi-stage mass analysis. In a quadrupole ion trap mass spectrometer, a quadrupole (four-rod) or an octapole (eight-rod) ion guide is placed before the three-dimensional quadrupole ion trap to improve the ion introducing efficiency.
A multi-stage mass spectrometer was proposed by M. G. Qian and D. M. Lubman in “A Marriage Made in MS”, Analytical Chemistry, vol. 67 (1995), No. 7, p. 234A. in which a three-dimensional quadrupole ion trap is placed before a time-of-flight (TOF) mass analyzer. In the mass spectrometer, a multi-stage mass analysis is first performed in the ion trap, and a high-resolution mass spectrum can be obtained with the TOF mass analyzer.
Thus by placing an ion storage device before another mass analyzer such as a TOF mass analyzer, various new mass analyzers have been developed. It should be noted however that, when ions are transferred from the ion storage device to the mass analyzer, the operation parameters of the ion storage device may affect the subsequent mass analyzer. For example, the radio frequency (RF) voltage used in the ion storage device for trapping or storing ions may change the initial kinetic energy of ions transferred to the mass analyzer.
In a mass spectrometer including a quadrupole ion trap and a TOF mass analyzer, for example, ions in the ion trap are always moving due to the RF voltage applied to it. When the ions are transferred to the TOF mass analyzer, an appropriate accelerating voltage is applied to the electrodes of the ion trap, and the ions are accelerated and injected into the TOF mass analyzer.
It is necessary to apply an adequate ion accelerating voltage to the electrode of the ion trap to which ion storing RF voltage was applied. But it is difficult to fix the ion accelerating voltage because an LC resonant circuit is connected to the electrode, which may change the ion accelerating energy. Since the ion storing RF voltage is appropriately modified according to the range of the mass to charge ratio of the object ions, the accelerating energy of ions injected to the TOF mass analyzer is affected according to the operating condition of the ion trap.
In U.S. Pat. No. 6,483,244, a method is described in which the RF voltage is decreased, or stopped, before ions are ejected from the ion trap. The method is now explained using FIG. 1. A coil 42 is connected to the ring electrode 11 to which the RF voltage is applied, and the overall inductance L of the coil 42 and other inductive elements and the overall capacitance C including that between the electrodes, that of the tuning circuit, that of the voltage detecting circuit, etc. form an LC resonant circuit. High voltage DC sources 44 and 45 with respective switching devices 46 and 47 are also connected to the ring electrode 11 to apply a bias voltage.
When the RF voltage to the ring electrode 11 is to be stopped, the two switching devices 46 and 47 are both turned ON, whereby the electric charge stored in the capacitance C of the LC resonant circuit is rapidly discharged. The voltage of the ring electrode 11 after discharge is determined by the voltage of the high voltage DC sources 44, 45 and the internal resistance of the switching devices 46, 47. Since the voltages of the two high voltage DC sources 44 and 45 are normally set equal, and the internal resistances of the two switching devices 46 and 47 are also normally set equal, the voltage of the ring electrode 11 after discharge is equal to the ground level. If, therefore, ions are ejected to the TOF mass spectrometer (TOFMS) 30 in such a condition, ions are properly accelerated irrespective of the operation condition of the ion trap 10, or specifically the magnitude of the RF voltage applied to it. This prevents deteriorating the performance of the TOFMS 30.